You may have also noticed that these piles of woodchips were once stumps. We had a pretty big stump outside our office where they took down an érable à giguère that had rotted in its core. I had watched that whole chopping up process from out my office window. A few days later, on the way back from lunch, I noticed this machinery in action:
It's some crazy stump destroyer. A big part of the operation is preventing chips and dust from flying all over the place. That's what those rubber flaps are for, as the direction of the grinding blade pushes the shrapnel behind and under it.
In concept, it is just like a giant grinder that goes back and forth across the top of the stump at an angle, digging deeper and deeper, coring the whole thing out and leaving ground up, presumably easily decomposable material.
I didn't get to watch them finish the job, so I don't know if it leaves a nice mound like that automatically or if there is a lot of shovelling and sweeping. I also don't know if they come back and get the chips or just let them sit there and slowly become part of the earth. Actually, if the pile remains there, I should take some to put on top of our work compost.